Leads implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life threatening arrhythmias, or to stimulate contraction (pacing) of the heart. Electrical energy is applied to the heart via the leads to return the heart to normal rhythm. Leads have also been used to sense in the atrium or ventricle of the heart and to deliver pacing pulses to the atrium or ventricle. The same lead used to sense the condition is sometimes also used in the process of delivering a corrective pulse or signal from the pulse generator of the pacemaker.
Cardiac pacing may be performed by the transvenous method or by leads implanted directly onto the ventricular epicardium. Most commonly, permanent transvenous pacing is performed using a lead positioned within one or more chambers of the heart. A lead, sometimes referred to as a catheter, may be positioned in the right ventricle or in the right atrium through a subclavian vein, and the lead terminal pins are attached to a pacemaker which is implanted subcutaneously. The lead may also be positioned in both chambers, depending on the lead, as when a lead passes through the atrium to the ventricle. Sense electrodes may be positioned within the atrium or the ventricle of the heart.
Pacemaker leads represent the electrical link between the pulse generator and the heart tissue which is to be excited. These pacemaker leads include single or multiconductor coils of insulated wire having an insulating sheath. The coils provide a cylindrical envelope, many times referred to as a lumen, which provides a space into which a stiffening stylet can be inserted. The conductive coil is connected to an electrode in an electrode assembly at a distal end of a pacing lead.
After the electrode assembly is positioned at a desired location within the heart, it is desirable to provide some method for securing the electrode assembly at that location. One approach is to use a passive device which has structure to allow for tissue growth surrounding the structure to affix the electrode assembly to the heart. Another approach is to use an active device where mechanical fixation devices are used to firmly anchor the electrodes in the heart. One type of mechanical fixation device used is a corkscrew, or a helix. During placement of the lead, the tip of the lead travels intravenously through veins and the heart. While traveling through the veins, the helix at the tip of the lead may snag or attach to the side wall of the vein. Since this is highly undesirable as it may cause damage or other complications to a patient, retractable helixes have been provided for leads.
The practitioner must maintain the electrode pressed against the wall of the cavity before shifting the screw. When the screw is shifted, the electrode may be correctly in contact with the wall, and the fixation screw, as it travels out of the body of the electrode, penetrates and becomes hooked in the tissue of the wall. Alternatively, the electrode may stop short of the wall of the cavity and it may be necessary for the practitioner to start again by retracting the screw and then turning the helix out again into the cardiac tissue. Thus, it is important for the helix to rotate freely within the electrode.
During use, the lead provides and receives critical information to and from the heart. The lead, therefore, must remain in sufficient operative condition without interference from entry of bodily fluids. To prevent entry of bodily fluids into the lead, a seal can be provided at the distal end of the lead. Conventional leads often use O-rings or puncture seals to seal the distal end of the lead from entry of bodily fluids. The O-ring seals can be difficult to manufacture due to dimensional constraints which also affects the extension/retraction mechanism of the lead, as well as the effectiveness of the seal. Puncture seals also may increase the difficultly of using the helix, since the helix needs to puncture the seal and the puncture seals can increase the friction between the extension mechanism and the seal. The friction makes it more difficult to extend or retract the extension mechanism and the helix. In addition, the structural integrity of the puncture seal can be jeopardized if the seal continues to tear from repeated movement and/or stress from the fixation screw.
Accordingly, there is a need for a lead which is sufficiently sealed from the environment. What is further needed is a seal which does not interfere with the extension and retraction of the helix.